1. Field of the Invention
The present invention relates to an apparatus for controlling the rotational speeds of drive motors provided in various sections in a bag forming/filling/sealing machine.
2. Description of Related Arts
Bag forming/filling/sealing machines are designed to form a film web into a tube, then fill therein articles to be packaged, and cut and seal the film tube. More specifically, the bag forming/filling/sealing machines form an elongated film web into a longitudinally extending tube, then feed a succession of equidistantly spaced articles to be packaged into the film tube, and cross-seal the film tube between adjacent articles, thereby forming a bag containing an article packaged therein.
A packaging machine of this kind includes at least a motor for transporting a film, a motor for transporting articles to be packaged, and a motor for pivoting a sealer. In such a packaging machine, the rotational speeds of the respective motors are controlled to time the rotations of the respective motors by means of a microcomputer.
A packaging machine disclosed in U.S. Pat. No. 4,712,357, for example, controls the rotational speeds of a film transportation motor and a sealer driving motor, based on the rotational speed of an article transportation motor which is kept constant. Another packaging machine disclosed in U.S. Pat. No. 4,909,018 controls the rotational speeds of an article transportation motor and a sealer driving motor, based on the rotational speed of a film transportation motor.
These packaging machines are each designed such that the reference motor stops at a predetermined original stop position and the other two motors each stop in a preset phase relationship with respect to the reference motor when an operation stop signal is generated. In the former packaging machine, for example, the film transportation motor and the sealer driving motor are each adapted to stop in a preset phase relationship with respect to the article transportation motor when the article transportation motor stops at the original stop position in response to an operation stop signal. At this time, a sealer is brought in contact with a film, resulting in fusion of the film due to heat applied thereto by the sealer.
In general, the stop position of a motor of this type is controlled by means of a computer. Therefore, it may be easy to control the stop positions of the respective motors so as to prevent the sealer from contacting the film. When the stop position of the sealer is changed due to a change in the length of articles to be packaged or a change in the interval between adjacent marks printed on the film, however, the contact of the sealer to the film is unavoidable.
To prevent the sealer from contacting the film, the rotational speeds of the film transportation motor and the article transportation motor should be controlled on the basis of the rotational speed of the sealer driving motor. It is easy to employ the rotational speeds of the film transportation motor and the article transportation motor as reference speeds because these motors are rotated at constant speeds. However, the sealer driving motor is rotated at a cyclic variable speed not at a constant speed. For this reason, it has been difficult to employ the rotational speed of the sealer driving motor as a reference speed.
To solve the aforesaid problem, it is an object of the present invention to control the rotational speeds of a film transportation motor and an article transportation motor on the basis of the rotational speed of a sealer driving motor to prevent a sealer from contacting a film when the operation of a packaging machine is stopped.